In many industrial processes, control of film thickness is of critical importance. For example, the manufacture of photographic film requires the generation of a uniform layer of emulsion on a backing. From the point of view of process control, it is advantageous to be able to measure the film thickness during the film generation process rather than measuring the film in a laboratory after the film has been manufactured. If samples are measured off-line, correction of any machinery malfunction cannot be performed until after a considerable volume of defective material has been processed. This leads to waste.
Methods for measuring the thickness of films using a Michelson interferometer are known to the art. For example, U.S. Pat. No. 3,319,515 to Flournoy describes the use of a Michelson interferometer for measuring the thickness of a film. In this system, the film is illuminated with a collimated light beam at an angle with respect to the surface of the film. The front and back surfaces of the film generate reflected light signals. The distance between the two reflecting surfaces is then determined by examining the peaks in the autocorrelation spectrum generated in a Michelson interferometer that receives the reflected light as its input.
While the arrangement taught by Flournoy is claimed to function adequately for the measurement of films, the apparatus taught therein is less than ideal. First, the Michelson interferometer must be located close to the film under measurement. The device taught by Flournoy utilizes a collimated light beam generated from a point light source. The amount of light available at the Michelson interferometer depends on the solid angle subtended by the Michelson interferometer as viewed from the light source. This solid angle decreases rapidly with distance; hence, both the source and Michelson interferometer must be very close to the film. Optical instruments such as the Michelson interferometer are poorly suited for location in manufacturing environments which tend to be dirty.
Second, the device taught by Flournoy is very sensitive to the angle of incidence of the light beam on the film. Small alignment errors can lead to errors in the thickness measurement. Even in instruments in which the incident light beam is perpendicular to the film surface, angular alignment errors can cause the reflected light to miss the entrance of the Michelson interferometer.
Third, prior art devices rely on calibrations done prior to the actual measurement to provide an accurate measurement of the position of the moveable mirror in the Michelson interferometer. Due to changes in mechanical linkages or thermal expansions and contractions, these calibration methods provide limited accuracy when used to measure thin films.
Fourth, prior art devices utilize white light sources such as heated filaments to generate the low coherence light signal that is directed at the film. Unfortunately, the amount of light reflected at the film boundaries is quite small. Since these light sources are limited in power, the sensitivity of measurements using this type of light source is limited by signal to noise considerations.
Broadly, it is the object of the present invention to provide an improved apparatus for measuring the thickness of a moving film.
It is a further object of the present invention to provide an apparatus that may be placed in a location remote from the film under measurement.
It is a still further object of the present invention to provide an apparatus that is less sensitive to angular alignment or longitudinal motion problems than prior art devices.
It is yet another object of the present invention to provide an apparatus that does not depend on calibrations performed prior to the actual measurements.
It is a still further objective of the present invention to provide an apparatus having a higher powered low coherence light source thereby providing better signal to noise ratios than obtainable with prior art white light sources.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.